Existing laser interference lithography systems use glass beam-splitters to split the original laser beam in free space. Normally, the original and subsequently split laser beams all propagate in free space. This setup requires extensive time for proper and accurate alignment. Changing interference patterns, especially altering their periods, requires realignment of the entire optical setup. Further, laser propagation in free space is deleteriously affected by environmental disturbances such as airflow or vibration.
Interference lithography is a method of fabricating sub-micron structures or arrays over a large scale (i.e. inches). Several types of regular and periodic patterns, including gratings, holes, pillars, cones and grids, can be produced after recording the interference of two or more coherent beams on photoresist. When a coherent laser beam is split into two or more beams that overlap within a certain area, there will be gratings or grids of regular light intensity patterns formed. These interference patterns, created by the split beam or beams, expose the photoresist, which subsequently “records” the interference patterns following development.
Unlike traditional optical techniques, laser interference lithography permits maskless configurations and rapid exposure on large-area substrates. Interference lithography can produce periodic nanostructures over a large area with high throughput and low cost, thus playing a key role in the emerging fields of energy production/efficiency, sensing, lighting, etc. The applications of periodic patterns include distributed feedback (DFB) lasers, field emission displays (FED), liquid crystal displays (LCD), advanced data storage applications, optical gratings, mythology standards and Moth-Eye subwavelength structures (SWS).
Interference lithography is the most common method to fabricate large-area, low-cost periodic structures. It is a well-known phenomenon that two coherent light waves can interfere, which can produce periodic patterns through two different schemes, i.e. the Lloyd mirror configuration and two-beam holography. While periods can be altered by changing the incident beam angle using a rotatable substrate holder, the Lloyd mirror configuration is restricted to mid-level pattern quality and a small exposure area, due to un-equal light paths and an imperfect reflective mirror. The two-beam holography setup has demonstrated higher pattern quality over a larger exposure area. However, altering the period in this configuration requires extensive realignment of optical components along each of the two split beams, which is very time-consuming and challenging, especially for periods less than 300 nm. Another challenge is that the two split beams can experience different environmental disturbances and air vibrations along their individual light paths, which induces phase noise that may blur interference patterns on the substrate.
CN103092002A describes a laser interference lithography system. However, CN103092002A uses discrete optical components to split and deliver the laser beams.
U.S. Pat. No. 6,522,433B2 describes an interference lithography technique utilizing holey fibers. The patent uses fibers with axially-formed holes to deliver laser beams for interference lithography. However, U.S. Pat. No. 6,522,433B2 still splits light beams in free space, and the aforementioned fiber is used in U.S. Pat. No. 6,522,433B2, increasing the cost. (In contrast, the potential patent uses fiber splitters for beam-splitting, and common polarization maintains single-mode fibers to deliver light).
U.S. Pat. No. 8,582,079B2 describes another interference lithography system. Nevertheless, in their technical scheme, light also propagates in free space, and discrete optical components are utilized to deliver and split light.
Sun, Y. L. et. al, ‘Lloyd's mirror interferometer using a single-mode fiber spatial filter’, Journal of Vacuum Science & Technology B, 2013. 31(2) reports an interference lithography system on the base of a Lloyd's mirror. However, their setup is manly used in low or medium-quality and small-area grating configurations, and cannot be used for large-area, high-quality lithography of periodic patterns.
Two-beam interference lithography in free space is used in large-area, high-quality and periodic pattern fabrication, but due to the inherent difficulty of assembly and frequent realignment of light beams, these systems are only utilized by major research institutes. These two-beam systems are not yet commercially viable for photolithography. Assembly, alignment and maintenance require professional, specialized skillsets, restricting commercial applications in the emerging areas of large-area, periodic nanostructure configurations.